1. Field of Invention
This invention relates to a valve mechanism which can be used as a purge valve for constant pressure sample cylinders.
2. Description of the Prior Art
Constant pressure sample cylinders are used worldwide to collect samples of various hydrocarbons. The construction and design of constant pressure sample cylinders is well known to those skilled in the art and is disclosed in patent application Ser. No. 07/243,589 filed on Sept. 12, 1988 and in prior U.S. Pat. Nos. 4,628,750; 4,463,599; 4,459,865; 4,403,519; and 4,172,670 assigned to Welker Engineering Company which are incorporated herein by reference. Constant pressure sample cylinders are manufactured by Welker Engineering Company and others.
In a typical constant pressure sample cylinder, a first end cap will be positioned on one end of the cylinder and a second end cap on the other end. The end caps are typically held in place by a plurality of elongate stud bolts which usually run the length of the cylinder and are joined by nuts on either end of the bolts. Inside the cylinder is a slidable piston which divides the cylinder into a first chamber for storage of sample and a second chamber referred to as the precharge chamber. The sample chamber communicates with a passageway in the first end cap. A sample inlet valve is connected to this first passageway to control sample flow. The precharge chamber communicates with a passageway in the second end cap. A precharge valve is connected to this second passageway to control flow of precharge fluid.
Prior to being taken to the field, cylinders are "precharged" in the laboratory. The precharge is usually equal to or slightly greater than expected pressures in the sample environment. For example, if pipeline pressure is approximately 500 psi, the second chamber will be pressurized to approximately 500 psi. This precharge moves the piston into contact with the first end cap and thereby effectively eliminating or reducing the volume in the sample chamber. There will always be a slight void between the piston and first end cap prior to collection of sample. As sample is collected the piston moves back against the precharge.
In a typical situation the constant pressure sample cylinder will be taken to an oil well for sampling of production. After the sample has been collected in the cylinder, it will be taken from the field to a laboratory for analysis. Crude oil is typically analyzed for numerous factors including but not limited to viscosity, specific density, sulfur content, etc.
Constant pressure sample cylinders are also used to a somewhat lesser degree to collect gaseous hydrocarbons such as natural gas. As will be known to those skilled in the art, natural gas at the wellhead typically contains methane, propane, butane, isobutane, natural gasoline and/or ethane. In certain conditions, such as production from a wet well, several of the constituents of natural gas may condense to form what is known as "light liquids" in the industry. When taking a natural gas sample, it is important that the liquids and gases remain in a steady state which is representative of the production from the well. A steady state means that the existing liquids do not vaporize and the existing vapors do not liquefy. Constant pressure sample cylinders have proven their usefulness for keeping natural gas samples in a steady state both during collection in the field and during transport from the field to a laboratory for analysis.
Traditionally natural gas was priced primarily by volume; however in more recent years the BTU content has become another important pricing consideration. The best way to accurately analyze the BTU content is to take a representative sample in the field and to make sure that the sample stays in a steady state during collection and delivery to the laboratory.
Calorimeters were initially used by laboratories for BTU analysis of a natural gas sample. Samples were typically collected in five gallon sample cylinders which were sometimes referred to as "bombs" by those in the industry. These types of cylinders do not contain an internal piston like the constant pressure sample cylinder. In order for the calorimeter to make an analysis of the BTU content of a sample it must burn for approximately fifteen minutes or more and will typically utilize ten to fifty cubic feet of natural gas from the sample cylinder. The calorimeter uses relatively large volumes of gas to operate and therefore older type sample cylinders needed to collect a relatively large volume of sample.
Most modern laboratories and pipelines now use gas chromatographs for analyzing the contents of natural gas. The BTU content is then calculated based on the content analysis from the gas chromatograph. Gas chromatographs may use one tenth of a milliliter or less of natural gas for purposes of analysis. It is therefore no longer necessary to collect large volume samples due to the improvement in the art of testing.
It has therefore become much more critical that the sample be as representative of the whole or possible. Any contaminant in the sample cylinder or in the sample itself could adversely affect the BTU analysis which dramatically affects the price of the natural gas.
It has therefore become common practice to purge a sample container prior to making a sample in the field. The purpose of purging the sample container is to eliminate any air which might be present in voids or passageways connecting to the sample container or in the sample container itself. Without an effective purge the collection of an accurate sample is virtually impossible.
Air shows up in a gas chromatograph primarily as nitrogen. Any excess amount of air trapped in passageways or voids and analyzed as nitrogen by the gas chromatograph could adversely affect the BTU analysis of the sample. It has therefore been recognized by those skilled in the art that any air trapped in passageways or in the sample container itself will adversely affect the representativeness of the sample. Prior to the invention of the present purge valve assembly there have been two common techniques for purging a sample container prior to sampling in the field.
The first prior art technique used a tee in connection with a purge valve as discussed hereinafter. Most sample stations in natural gas pipelines contain one or more valved outlets connected to probes in the pipeline. The initial step in the purging process is to open the valve connected to such a probe to verify open communication with the pipeline. When the natural gas is vented to atmosphere in this fashion the operator is certain that the probe is not plugged. Opening a valve in this manner will also disburse any light liquids which may be entrapped in the probe or the valve. The valve is then piped to a tee having a purge valve on one side of the T and the inlet valve to the constant pressure sample cylinder on the other side. The inlet valve on the sample side of the constant pressure sample cylinder is opened, the purge valve at the tee is closed and the valve at the pipeline is opened. This allows pipeline pressure which is typically in excess of 500 psi to reach through the connective tubing, through the tee, through the inlet valve on the cylinder, through the passageways in the end cap on the constant pressure sample cylinder and into the sample chamber in direct contact with the piston. Although this circuit is pressurized with pipeline pressure, there is no true fluid flow because there is no outlet from the sample chamber. The piston does not move in this situation because the precharge side of the constant pressure sample cylinder has been prepressurized to equal or exceed pipeline pressure.
The next step in this type of purging process is to close the valve on the pipeline and open the purge valve at the tee. This vents the pipeline pressure in the connective tubing and the cylinder to atmosphere; again there is no true flow because it is a closed circuit. This purging process is then repeated three to five times to hopefully eliminate all air from the connective tubing, the inlet valve on the sample cylinder, the passageways in the end cap and the void between the piston and the end cap in the sample cylinder. Because the piston has been driven into contact with the end cap the void between the piston and the end cap is practically diminimus; however, when dealing with gases, very small areas could retain enough air or other contaminants to adversely affect the sample. At lower pipeline pressures of fifteen to twenty pounds the purging process should be repeated approximately fifteen to seventeen times. After the purging process is complete, the constant pressure sample cylinder is disconnected from the piping and connected to a sampler for collection of a sample.
This old technique of purging has a significant disadvantage because the purge valve is connected to a tee which is located between the pipeline and the constant pressure sample cylinder. There is therefore no direct flow through the inlet valve on the constant pressure sample cylinder or the passageways in the end cap or the void formed between the piston and the end cap. This lack of direct flow was recognized by Applicant as a significant disadvantage which needed to be corrected in order to improve sampling techniques.
Applicant therefore developed a second prior art technique using a purge valve in a different location. Applicant began to drill additional passageways in the end cap of its constant pressure sample cylinder and install hardface valves in these passageways. This repositioned valve provided an outlet to the circuit to create a true flow when purging. Typical valves used by the Applicant for this purpose include Whitey valve model no. SS-20KM4F4 and Manifold Fabricators valve model no. MNV-MF-25-316S. This outlet connected to the sample chamber made a significant improvement in the purging process by allowing gases to actually flow through the inlet valve, the passageways in the end cap and the void between the piston and the end cap.
This improved purging process requires that the inlet valve to the constant pressure sample cylinder be connected to the pipeline; however there is no need for an intermediate tee or a purge valve at the tee. The purge valve is directly attached to the end cap. In this improved purging process the inlet valve to the constant pressure sample cylinder would be opened allowing pipeline pressure to reach through the inlet valve, the passageways in the end cap and into the void between the piston and the end cap. The inlet valve would then be closed and the purge valve would be opened thus venting the fluids to atmosphere. The purge valve would then be closed and the inlet valve would be opened. Repetition of the aforementioned steps in this improved purging process actually allows natural gas to pass from the pipeline through the inlet valve, the passageways in the end cap, the void between the piston and the end cap and out through the purge valve to atmosphere. This improved purging method virtually eliminated any air from the sample.
Unfortunately because of the added cost for an additional hardfaced purging valve this improved purging process has not been widely adopted by industry. In order to promote greater sampling accuracy and encourage use of this improved purging technique applicant has invented a low cost purge valve which can be directly installed in the end cap and used in lieu of prior hard face purge valves which were relatively much more expensive.